Conventionally, there is known a method for producing a three-dimensionally shaped object by irradiating a light beam on a powdery material, which method is usually referred to as “Selective Laser Sintering or Melting”. In this method, the three-dimensionally shaped object is produced by repeating (i) a step of irradiating a light beam on a specified portion of a powder layer to sinter or melt the same into a solidified layer and (ii) a step of placing a new powder layer on the solidified layer and irradiating the light beam on a specified portion of the new powder layer to form another solidified layer (see, e.g., Japanese Patent Laid-open Publication Nos. H1-502890 and 2000-73108). In case where a metal powder is used as the powdery material, the three-dimensionally shaped object thus produced can be used as a mold for molding a plastic article. In the event that a resin powder is used as the powdery material, the three-dimensionally shaped object thus produced can be used as a plastic article. This production method enables a three-dimensionally shaped object of complex shape to be produced within a short period of time.
In order to avoid oxidization of the three-dimensionally shaped object, the production thereof is performed within a chamber kept in a specified inert atmosphere. Installed inside the chamber are a powder layer forming unit, a shaping table on which the powder layer and/or the solidified layer are placed, and so forth. A light beam irradiating unit is installed outside the chamber. The light beam emitted from the light beam irradiating unit is irradiated on a specified portion of the powder layer through a light transmissive window of the chamber. For example, as can be seen in FIGS. 1 and 13, the chamber 50 is provided with a light transmissive window 52 through which the light beam L can be irradiated into the chamber 50.
When the light beam is irradiated on a powder layer to sinter or melt a powder, smoke-like substances called fumes 60 (e.g., a metal vapor or a resin vapor) are generated from the beam-irradiated portion as shown in FIG. 1. The fumes move upwards to adhere to the light transmissive window or burn, which may reduce the light beam transmittance of the light transmissive window. Reduction in the light beam transmittance makes it impossible to obtain a desired solidified layer and to produce an intended shaped object. In case where a metal powder layer is used as the powder layer, the reduced light beam transmittance results in failure to stabilize sintering or inability to increase sintering density. This poses a problem in that the three-dimensionally shaped object shows a decrease in strength.
In addition, the fumes may directly affect the light beam irradiated into the chamber. More specifically, the fumes may move upwards and may sometimes interrupt the light beam path, thereby reducing the irradiation amount of light beam (namely, the amount of light beam irradiated on the powder layer). In other words, there is a fear that, due to the interruption of the light beam path by the upwardly moving fumes, the energy quantity of light beam delivered to the powder layer may be substantially smaller than a specified value.